A hot water heater, as contemplated herein, typically comprises a permanently enclosed vertically mounted cylindrical water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating material in at least a portion of the annular space for providing thermal insulation to the water tank. Polymer foam expanded directly within the annular space is an effective insulating material. Particularly, polyol and isocryanate reactants with foaming agents are commonly reacted in place within the annular space to form polyurethane foam which readily expands to fill the available space and rapidly cures into a hard, closed cell foam insulating material. Means, such as foam dams, are provided for containing the expanding foam within the annular space.
The water tank has various appurtenances such as inlet, outlet and drain fittings. Especially, the water tank is provided with water heating and temperature control means. Typically for electrically heated water heaters, the water heating means comprises an electrical resistance heating element. The heating element extends through a fitting in the wall of the water tank such that the resistance heating element is inside the tank and means for connecting the resistance heating element to an electrical power source is outside the water tank.
Temperature control means for an electrically heated water heater commonly comprise a mechanical thermostat which operates a switch to allow electrical power through the electrical resistance heating element when water in the tank is sensed to be below a selected set point temperature, and operates the switch to stop electrical power from passing through the electrical resistance heating element when the water in the tank is at or above the set point temperature. With such temperature control means, electrical power through the electrical resistance heating element is either fully on, passing full electrical current, or completely off.
The mechanical thermostats commonly used for water heaters comprise a bimetallic disc of metal which is mounted in contact with the outer wall of the water tank, for sensing temperature of water in the water tank. The bimetallic disc, when cooler than the set point temperature, is flat. However, at the set point temperature, the bimetallic disc snaps into a convex dome shape which the disc maintains until it cools to a temperature below the set point. At a temperature below the set point, the bimetallic disc snaps back into a flat position. Mechanical hysteresis in the bimetallic disc causes the temperature at which the disc snaps back into flat position to be substantially lower, (5 to 10.degree. C.), than the set point temperature at which the bimetallic disc snaps into a convex dome shape. The bimetallic disc is connected, by an electrically insulated push rod, to an electrical switch in a phenolic resin housing which is attached to the outer wall of the water tank. The electrical switch is in an electric line connecting the electrical resistance heating element to a source of electric power.
A threaded stud is mounted in the housing in contact with the bimetallic disc for manual adjustment of tension on the bimetallic disc such that the bimetallic disc may be adjusted to snap from flat position to convex dome position at the desired set point temperature.
When the bimetallic disc senses a cool water temperature, and the disc is in a flat position, the push rod closes the electrical switch, allowing electrical power to flow through the electrical resistance heating element, thus heating water in the water tank. As the water heats, the bimetallic disc snaps into a convex dome shape at the set point temperature and the push rod opens the electrical switch, shutting off power to the electrical resistance heating element. The electrical switch remains open, and the power cut off, until water in the tank cools enough for the bimetallic disc to snap into a flat position, thus closing the electrical switch and allowing electrical power to flow into the electrical resistance heater. The electrical power is either on at full capacity of completely off.
Due to variations in manufacture and hysteresis of the bimetallic disc, the temperature at which the disc will snap from a flat position to a convex dome position can vary by as much as 25.degree. F. (14.degree. C.). Tension may be applied to the bimetallic disc with the threaded stud for adjusting the set point temperature at which the bimetallic disc will snap from a flat position to a convex domed position. The threaded stud is not calibrated, and adjustments of tension to achieve action of the bimetallic disc at a selected set point temperature are made by trial and error. Often a mechanical temperature control, employing a bimetallic disc thermocouple, will "overshoot" the desired set point temperature, allowing the electrical resistance heating element to continue heating water in the water tank above the desired set point temperature.
The mechanical temperature control means, described above, and the electrical resistance heating element are in contact with the outer wall of the water tank and extend into the annular space between the water tank outer wall and the shell. These items must be accessible for servicing. One or more openings in the wall of the shell provide access to the temperature control means and the electrical resistance heating element. A dam structure is provided around each opening for protecting the temperature control means and electrical resistance heating element from contact with the insulating foam and to prevent the foam from escaping the annular space via the openings in the shell wall.
Electrically heated water heaters are provided with safety devices. A high temperature safety shut off switch is installed in the electric power line which cuts off power to the electrical resistance heating element when the temperature in the water tank rises above a safe level. Also, the water tank is provided with a high temperature relief valve which is set to open at about the water boiling point, (100.degree. C.), for preventing an increase of pressure in the water tank from boiling water.